1. Field of the Invention
The present invention relates generally to a method of forming minute patterns and, more specifically, to a method of forming minute patterns by using a chemically amplifying type resist.
2. Description of the Background Art
A study on the performance of negative Microposit SAL601-ER7 resist (provided by Shipley Company, a novolak resin based electron beam resist) having both high sensitivity and high contrast has been reported (J. Vac. Sci. Technol. B6 (6), Nov/Dec 1988).
FIG. 4 shows a process for forming patterns by using the Microposit SAL601-ER7 described in the report mentioned above. Referring to FIG. 4, features of the Microposit SAL601-ER7 will be described.
The resist film of Microposit SAL601-ER7 comprises novolak resin, radiation sensitive acid generator, and a cross-linking agent. The resist film has a large difference between the dissolution rates of the unexposed and exposed areas of the film in aqueous alkaline developer. The magnitude of the dissolution rate of the resist film is determined by its degree of cross-linking. Upon exposure to the electron beam, the radiation sensitive acid generator releases acid into the film in proportion to the dose. After exposure, there is little difference between the dissolution rates of the exposed and the unexposed areas of the film. An image is formed in the film when the resist receives a post exposure bake because the acid catalized cross-linking step requires thermal energy. The degree of cross-linking of the exposed area is governed by both the dose and the post exposure bake conditions. If more acid is present in the film, less thermal energy is required to generate a given percentage of cross-linking. A resist having such features is called a chemically amplifying type resist.
Though Microposit SAL601-ER7 having the above described features is an electric beam resist, it is also sensitive to the deep UV range. The deep UV ray is used for forming minute patterns in lithography. If a technique for sensitizing the Microposit SAL601-ER7 with the deep UV ray is established, more minute patterns will be formed.
The inventors performed experiments to form minute patterns by using Microposit SAL601-ER7 as a resist and KrF excimer laser beam (having the wavelength of 248 nm) as a radiation beam. FIGS. 5A to 5E schematically show the results of the experiment in cross sections. Referring to FIG. 5A, a silicon substrate 1 is prepared. Referring to FIG. 5B, the Microposit SAL601-ER77 is applied on the silicon substrate 1 by spin coating, thereby forming a resist film 2 having a prescribed thickness. Thereafter, baking is performed for 60 to 80 sec at 90.degree. to 100.degree. C. prior to exposure, in order to vaporize the solvent.
Thereafter, referring to FIG. 5C, selective exposure to the KrF excimer laser beam 6 is carried out using a photomask 5, by reduction type projection printing. Consequently, exposed areas 2a and unexposed areas 2b are provided in the resist film 2. In the exposed areas 2a, H.sup.+ ions which are the catalyst of the cross-linking reaction (simply represented by the sign + in the figure) are generated. Referring to FIG. 5D, the exposed resist is baked for 60 to 180 sec on a hot plate 4 heated to 80.degree. to 140.degree. C. By the catalyst of the H.sup.+ ions, cross-linking reaction of the novolak resin occurs in the exposed areas 2a.
Referring to to FIG. 5E, development is carried out by using 2.38% tetramethyl ammonium hydroxide solution, then the unexposed portions are dissolved in the solution, and negative patterns 7 are provided as a result.
However, the profile of the resist 7 is tapered with the lower parts being narrower, as shown in FIG. 6B (which is a partial expansion of FIG. 5E). When the underlying layer is processed by, for example, reactive ion etching by using the pattern 7 having such a resist profile, the dimension cannot be well controlled.
The reason why the resist profile is tapered with the lower portion being narrower as shown in FIG. 6B is that the novolak resin included in the resist absorbs the KrF excimer laser beam well. More specifically, referring to FIG. 6A (which is a partial expansion of FIG. 5C), the KrF excimer laser beam 6 is much absorbed on the surface of the resist film 2, so that more H.sup.+ ions are generated in the upper portion of the resist film 2, and the concentration of the H.sup.+ ions becomes lower in the lower portion. In addition, H.sup.+ ions are diffused in the lateral direction by heat. Therefore, when PEB is performed, the H.sup.+ ions occupy a larger planar area in the upper portion of the resist and occupy a narrower area in the lower portion, as shown in FIG. 6A. Since the cross-linking reaction occurs where there are H.sup.+ ions, the tapered resist profile with the lower portion being narrower is provided after development, as shown in FIG. 6B.